1. Field of the Invention
This invention relates generally to the serial transfer of data from a source to a data sink, and more particularly to a means for the serial transfer of data in a rate-independent fashion.
2. Description of the Prior Art
Means for the serial transfer of data in a rate-dependent fashion are well-known in the art. All of these rate-dependent means are characterized by the fact that the data receptor must be aware of the rate at which the data dispatcher is transferring data. The receptor uses the element of time to discriminate between different pieces of information signaled by the data dispatcher.
Typical of rate-dependent signalling means are exemplified by DC signalling and AC transmission as described in Chapters 6, 7 and 8 in a text entitled "Telecommunications and the Computer" by James Martin (Prentice-Hall, Englewood Cliffs, N.J., 1969). In DC signalling, data is transferred by two logic levels, generally referred to as marks and spaces. The marks and spaces are coded to represent letters and digits. A code for letters and numbers might be designed with five bits of information (a mark or a space being a bit) representing a letter or number. The element of time and rate-dependence enters into DC signalling when it is realized that two consecutive bits of information having the same value (say mark bits) will have the same logic level (say high logic value, perhaps +5 volts). Thus, to determine that there are two different bits being transferred, a time frame is defined in which the value of the data dispatcher's signal is sampled. In the case of two marks, then the two consecutive samplings by the data receptor would both detect high logic levels. By introducing the time frame element, rate dependence of the signalling means is introduced. This is because a receptor's sampling frequency, say 1000 samples/second, clearly dictates that for proper reception of the data that it must be dispatched at the same rate, in this case 1000 bits of information/second.
In DC signalling, the fact that the data dispatcher and data receptor will not have exactly the same rate because of impreciseness of physical components is accommodated to a small degree by ending coded characters (of say 5 bits) by a Stop Bit. Also, a Start Bit, which is usually a space, denotes the beginning of a character and signals the receptor to sample the desired number of bits. The Stop Bit is usually a mark. The Stop Bit is somewhat longer in time and by following the coded character, allows the data receptor to detect the end of the character even if the receptor's sampling has been lagging in the time frame. The Stop Bit is typically 1.5 times the duration of marks and spaces in the coded-character.
AC signalling is an extension of the principles of DC signalling. AC signalling involves the modulating of the DC marks and spaces for long-distance signal transmission and then the demodulation of those signals back into marks and spaces at the receiving end. Frequency-shift keying is a frequently employed AC signalling technique.
There is needed a signalling means which a data dispatcher can adjust the data transfer rate without prior notification to the receptor.